SOL-GEL process for the preparation of vitreous films possessing high adhesion properties and stable colloidal solutions

ABSTRACT

Process for the preparation and the deposition of vitreous films on substrates comprising: the dissolution of one or more metal alkoxides in aprotic solvent, the hydrolysis of the resulting solution, the eventual removal of the formed alcohol, the deposition of the resulting sol on the substrate surface of interest and the final drying of the film.

INTRODUCTION AND BACKGROUND

The present invention relates to a sol-gel process for the preparationof vitreous films characterized by high adhesion on the substrate ofinterest, to the vitreous films obtained thereby, and to the colloidalsuspensions obtained in the initial phase of the above mentionedprocess, that can be removed and maintained as such for indefiniteamount of time, before being processed again to achieve the filmdeposition.

More particularly, the object of the present invention is a process forthe preparation and the deposition, on the surface of suitablesubstrates, of vitreous films comprising the operations of dissolvingone or more metal alkoxides in an aprotic solvent, of adding controlledamounts of an aqueous solutions of a catalyst to the solution therebyobtained, of monitoring with adequate precision the hydrolysis reaction,of the eventual extraction and removal the amount of alcohol produced inthe hydrolysis reaction, of the gelation of the sol on the surface ofinterest and of the final drying of the film. It is of particularimportance in this invention, the possibility to stop the process afterhydrolysis and before film deposition to isolate a sol that in theapparent condition of a clear and stable solution, can be stored safelyfor long time at room temperature.

The vitreous films are used in the field of telecommunications onphysical carriers for both optical and electronic applications; becauseof their remarkable resistance to temperature, abrasion and corrosion,silica films are used, as an example, in processes typical of theelectronic industry as surface planarizers in a semiconductor afterprevious treatments have created off-plane circuitry. Similar silicafilms are also used to produce electrical insulation betweenintracircuitry elements or between conductors of different circuits on asemiconductor chip.

Moreover [A. S. Holmes et al., “Applied Optics”, Jan. 9, 1993, vol. 32,n. 25, pages 4916 -4912], silica films find applications asantireflexion and as planar wave-guides, or even as host material foractive dopants for nonlinear optic as well as optical sensingapplications.

Many methods were developed for the preparation of optical devices [A.S. Holmes et al., ibidem], among other, as an example, can be called thethermic oxidation of silicon, or the technique of deposition underreduced pressure, known as “sputtering”, chemical vapor deposition, etc.

The technologies to which the above-cited methods refer are, however,relatively complex and sophisticated, their practical exploitationrequire specialized equipment and time-consuming procedures that resultin relatively costly operations.

Moreover, the limited productivity to which often they are couplederodes the convenience for their industrial application.

It is a consolidated conclusion [A. S. Holmes et al., ibidem; Rui M.Almeida, “International Journal of Optoelectronics”, 1994, vol. 9, n. 2,pagg. 135-142], that the more promising technology for large scaleproduction of vitreous films is the sol-gel technology. Such atechnology is based on three fundamental steps:

-   a. sol formation in alcoholic medium (particle suspension or    dispersion in a liquid),-   b. gelation or transformation of sol by condensation into gel (a    solid skeleton inglobating a continuous liquid phase),-   c. drying of the gel.

In the procedure for vitreous film deposition on substrate, the sol isapplied to the surface to be coated and the gel is formed as consequenceof the solvent evaporation; a drying phase by simple heating completethe coating procedure.

It is however known that in the production of vitreous films, eventhrough sol-gel based technology one have to face limitations, speciallyin the thickness of the films obtained and/or in the tendency of suchfilms to crack, often in the drying phase.

Solutions to the problem were promptly sought and some of technicalanswers reported in this field by the scientific literature and byrecent patents, with no doubt contain preludes to future industrialapplication. Examples of such important reports are following:

-   -   Italian patent application n. NO 98 A 000004 describes a sol-gel        process for the preparation of thick films of silicon-dioxide        that improves the well-known procedure by the addition of a        certain quantity of fumed silica to the hydrolysis product of        silicon alkoxide.    -   A. S Holmes et al. already quoted article describes a process of        high-temperature consolidation of a multilayers film.    -   U.S. Pat. No. 6,130,152 describes a sol-gel process that teaches        to add a combination of two solvents with different boiling        point to the hydrolyzed solution (tetraethylorthosilahe,        ethanol, water and acid).    -   U.S. Pat. No. 6,017,389 describes the preparation of siliceous        films starting from the combination of tetraethylorthosilane and        silica in anhydrous ethanol that is hydrolyzed by aqueous        ammonia and necessitate of a very high thermal treatment at the        end.

All the known solutions, of which the precedent paragraphs are only afew examples, make the sol-gel technology attractive for the preparationof vitreous films, but do not offer a fully convenient process for abroad industrial application either because of the temperaturerequirements or because of thickness, or for particular technicalitiesthat could negatively influence the industriability of the process.

The Applicant has now found that it is possible to prepare and todeposit on substrates, according to the sol-gel technology, vitreousfilms following a procedure presenting none of the disadvantages of theknown art, nor the limitations to general applicability so far reportedon count of sol-gel technology.

SUMMARY OF THE INVENTION

As a matter of fact it is a first object of the present invention aprocess for the preparation and the subsequent deposition on suitablesubstrates of vitreous films comprising the following steps:

-   -   Preparation of a solution in an aprotic solvent of one or more        alkoxides having the general formula        X_(m)—Me—(OR)_(n−m)    -    where Me is a metal belonging to groups 3°, 4° or 5° of the        Periodic System of the Elements; n is the Me valence; X is R₁ or        OR₁, with R₁ equal to or different from R, m is zero or an        integer number equal to or lower than 3; R and R₁ are        hydrocarbon radicals with a number of carbon atoms up to 12.    -   Hydrolysis of the obtained solution in the presence of a        catalyst by addition of water.    -   Eventual removal of the alcohol formed during the hydrolysis        reaction.    -   Deposition of the sol on the substrate of interest.    -   Final drying and stabilization of the film characterized by the        fact that the preparation of the sol occurs in an aprotic        medium.

DETAILED DESCRIPTION OF INVENTION

The film deposition can be carried out immediately after completing thehydrolysis reaction, or the colloidal dispersion obtained in this phasecan be removed, whatever be the level of hydrolysis reached and storedindefinitely to be used at a desired time: this peculiar solution ischaracteristic of the process of the current invention and also definethe second object of the invention as just here stated: to provide astable sol constituted by the hydrolysis products of an alkoxidecorresponding to the above-reported formula, dissolved in an aproticsolvent, from which eventually the alcohol produced by the hydrolysisitself would have been removed. On this ground are important aspects ofthe process of this invention the following:

-   -   simplicity of preparation of the sol composition and stability        thereof in time at room temperature;    -   easiness of film casting, according to known techniques executed        under mild conditions and with acceptable times;    -   rapid gelation times, with film remaining homogeneous and the        mechanical characteristics of the substrate remaining unchanged;    -   not need for post-treatments.

As a consequence of such a process the films, also object of the presentinvention, are characterized by:

-   -   high adhesion to substrate;    -   final shrinking without crackings;    -   good mechanical and insulating properties;    -   good planarization capacity on the substrate surface;    -   good optical properties.

Referring to the process for the preparation vitreous films according tothe present invention, in the above alkoxide formula the “metal” canpreferably be silicon and, among all possible alkoxides, particularlysuitable to the purposes of the present invention can be the following:

-   -   tetramethylorthosilicate    -   tetraethylorthosilicate    -   tetrapropylorthosilicate    -   tetrabutylorthosilicate    -   ethyltriethoxysilane    -   methyltrimethoxysilane    -   methyltriethoxysilane

The alkoxide or mixture of alkoxides can be dissolved in a suitablesolvent and reacted with controlled amounts of water in the presence ofa catalyst, preferably of acid or basic character: the reaction can beaccomplished, under agitation, at room temperature, for a time includedbetween few minutes and few hours. The medium is an aprotic compound andcan be preferably chosen among acetone, tetrahydrofuran, dioxane; thealkoxide can be dissolved in such a solvent or mixture of solvent at aconcentration between 30 and 60% by weight.

Hydrolysis reaction can be carried out through addition of controlledamounts of water, so to maintain the molar ratio H₂O/Me between 0,5 and5, preferably between 1,5 and 4 and, even more preferably between 2 and3. As far as the acid catalyst is concerned, this might be any mineralor organic acid with Ka between 0,1 and 3.

The basic catalyst can be ammonia, other amines suitable for the controlof pH in the sol, or other suitable to the purpose.

According to a preferred embodiment of the inventive process, thehydrolysis can be conducted in the presence of aqueous molar solution ofHCl.

The molar ratio between alkoxide and acid can range from 1/0,001 to 1/1,but is preferably that such a ratio be maintained between 1/0,1 and1/0,01. At the end of the hydrolysis reaction, the product has theappearance of a clear liquid, free from solid particles due to traces ofinsoluble gels. It is possible at this point, according to an originaland innovative aspect of the present invention, to obtain a sol withimproved stability by elimination of the alcohol formed in thehydrolysis reaction.

The removal of the alcohol can be accomplished following any methodsknown to the state of the art. To the purpose of exemplification theApplicant reports the experimental event that in the case of ethanolproduced by the hydrolysis reaction, this is removed subjecting the solto a controlled partial desolventization in constant evaporation underreduced pressure, keeping the colloidal suspension at a temperature ofabout 40° C.

As previously mentioned, the process according to the present inventioncan be interrupted in its course after hydrolysis, thus obtaining a solthat is an object and an integral part of the present invention,characterized by high stability and such as to allow its long timestorage without deterioration or contamination by insoluble precipitatesthat will compromise any further use thereof. The storage andconservation of such sol can simply be at room temperature.

The final sol so obtained after the hydrolysis reaction or afterstorage, is used for the deposition of the films having thecharacteristic properties already described. Deposition on the desiredsubstrate does not present any particular difficulty and can be carriedout according to the various techniques used in the known art as knifedeposition, dip-coating, spin-coating. An important property of the solaccording to the present invention is that of join to its high shelfstability its short time of gelation once deposited on the substrate.Referring, as an example, to the deposition for spin-coating the filmsare deposited in seconds using rotational speed of the order of1500/2500 rpm and they do not any washing or post-treatments. The coatedsubstrates can be moved immediately because the instant-gelation makesit solid at the time of removal from the spin-coater. In fact it is animportant property of the film according to the invention to demonstrateexcellent adhesion with regard to the majority of the substrates used inindustry as, for example, silicon semiconductor wafers, galliumarsenide, polycrystalline silicon, glass, quartz glass etc.

The final drying of the film has the purpose to completely eliminate theresidual solvent from the gel and to complete the film stabilization; itcan be conducted setting the film-substrates in an oven at temperaturesbetween 80° C. and 500° C. The operation is completed in 10-20 minutestime-span and is occurring without the appearing of inhomogeneities anddefects on the film due to impurities and or cracking caused byexcessive shrinking of the gel in the drying phase.

According to an alternative embodiment of the inventive process,separate alkoxide solutions can also be prepared the same undergoinghydrolysis at different times. The distinct sols obtains thereby can beused to deposit films on different samples of the substrate of intereston different substrates as well as on the same substrate according to adeposition order suggested by the skillness or the technological needs,or they can be recombined into one sol of more integrated properties forbetter matching the specifications required in the deposited films.

Finally we should underline that in the process according to the presentinvention there are no problems concerning film-thickness and it ispossible to obtain any value of thickness, between 10 nm and 2 nm. Thefinal thickness is controllable by monitoring concentration of networkprecursor into the sol as well alkoxide or alkoxide mixtures typesentered. The Applicant has determined that alkoxides corresponding tothe above formula with X equal to R₁ contribute thickness higher thanthe ones of the film obtained via alkoxides where X is equal to OR₁:special formulations of sol that controls film thickness through the useof different type of alkoxides is not only possible but, at times verypractical within the frame of the process of the present invention.

In the following are reported some achievements in the field of presentinvention only for the purpose of supplying practical examples withoutany limitation of the invention itself to those embodiments.

EXAMPLES Example 1 Preparation of a Sol Based on Tetraethylorthosilicate

A 1l. round flask containing a magnetic stirrer bar is loaded with 266 gof an anhydrous acetone and 156,8 g (0,75 moles) oftetraethylorthosilicate (TEOS). Keeping the flask at room temperature,under constant stirring 32,4 g of HCl 1M aqueous solution are added byslow dropping (molar ratio TEOS: H₂O: HCl=1: 2,3: 0,016). The additionof water requires about 15 minutes. During this time the temperaturerises from 20° C. to 40° C.

Stirring of the mixture is maintained for about 15 minutes, then fromwhat is now a clear liquid a volume of 50 c is collected and stored in aglass container with screw stopper (Solution A).

With the same procedure an identical round flask is loaded with 266 g ofanhydrous ethanol. The same procedure used to prepare the acetone sol(solution A) is now used to prepare an analogous sol in ethanol(solution B) both solutions are left on the laboratory bench.

After 2 days solution B shows clear sign of gelation. Solution Amaintains its original status after 1 month without any indication ofgelation.

Example 2

266 g of anhydrous dioxane and 156,8 g (o,75 moles) oftetraethylorthosilicate (TEOS) are loaded on 1l. round flask containinga magnetic stirrer bar. Following the experimental procedures of example1, 41,6 g of 1M aqueous HCl are added (molar ratios TEOS: H₂O: HCl=01:2,32: 0,016).

After 30 minutes a fraction of liquid (50 ml) are removed from the flaskand stored in a glass container with screw cap and called Solution C.The remaining solution is transferred to a rotating evaporator andsubjected to evaporation at a reduced pressure (about 100 tor) for about20 minutes, keeping the internal temperature at 5° C. and collectingabout 80 ml of evaporated liquid. The evaporation is interrupted and anequal volume of dioxane added to replace the ethanol solution removed.The evaporation is restarted under the same conditions. In parallel, theconcentration of ethanol in the evaporate is determined by gaschromatography. The operation of evaporation and replacement ofevaporate with dioxane is repeated until the chromatographic analysis ofthe collected liquid shows 99% extraction of ethanol generated inhydrolysis. A sample of 50 ml of the sol so treated is transferred to aglass container with a screw cap and stored as Solution D. Solution Cshows clear signs of gelation after 30 days, while Solution D maintainsits original condition after 365 days without sign of gelation.

Example 3

177 g of anhydrous dioxane, 122 g (0,588 moles) of TEOS and 68,4 g(0,384 moles) of methylthreethylorthosilicate (MTEOS) are introduced ina 1l. round flask. Following the procedures of example 1, 41,6 g of HCl1M aqueous, solution are added (molar ratios TEOS: MTEOS: H₂O:HCl=1:0,65: 2,22: 0,0416). After 30 minutes a fraction of 50 ml of the liquidis removed, stored in a glass bottle with a screw cup and labeledSolution E. The remaining liquid is set into a rotating evaporator andevaporated with the procedures of example 2 completing 3 cycles ofevaporation and dioxane addition.

The chromatographic analysis of the evaporated solvents indicate that99% of the ethanol produced in hydrolysis has been recovered. A sampleof 50 ml of the liquid remaining in the evaporator flask (sol) isremoved, stored in a glass bottle with a screw cup and labeled SolutionF. Solution E shows clear signs of gelation after 30 days while solutionF maintains its original condition without signs of gelation after 365days.

1. A process for the preparation and the deposition of a vitreous filmon a substrate comprising: preparing a solution in an aprotic solventselected from the group consisting of acetone, tetrahydrofuran anddioxane of at least one metal alkoxide corresponding to the formulaX_(m)—Me—(OR)_(n−m) where Me is a metal selected from the groupconsisting of Group 3°, 4° and 5° of the Periodic Table of Elements; nis the valence of Me; X is R₁ or OR₁, R₁ is the same as or differentfrom R, m is either zero or integer number equal to or lower than 3; Rand R₁ are hydrocarbon radicals with a number of carbon atoms equal toor lower than 12; adding a catalyst to the solution; hydrolyzing thesolution in the presence of the catalyst in a hydrolysis reaction byaddition of water to maintain a molar ratio of H₂O/Me between 0.5 and 5to obtain a clear hydrolysis reaction product; removing alcohol formedduring the hydrolysis reaction under reduced pressure and forming a solin the presence of the aprotic solvent wherein an equal volume of theaprotic solvent replaces the alcohol removed; depositing the hydrolysisreaction product in the form of a sol on the substrate; optionallydrying and stabilizing of the vitreous film.
 2. The process for thepreparation and deposition of a vitreous film onto a substrate accordingto claim 1 wherein the alkoxide is selected from the group consisting oftetramethylorthosilicate, tetraethylorthosilicate,tetrapropylorthosilicate, tetrabutylorthosilicate, ethyltriethoxysilane,methyltrimethoxysilane, methyltriethoxysilane, and mixture thereof. 3.The process for the preparation and deposition of a vitreous film onto asubstrate according to claim 1 where the solution of the alkoxide ormixture of alkoxides in the aprotic solvent is from 30% to 60% byweight.
 4. The process for the preparation and deposition of a vitreousfilm onto a substrate according to claim 1 wherein hydrolyzing of thealkoxide is accomplished by adding a controlled quantity of water. 5.The process for the preparation and deposition of a vitreous film onto asubstrate according to claim 1 where the ratio H₂O/Me is from 1.5 to 4.6. The process for the preparation and deposition of a vitreous filmonto a substrate according to claim 5 where the ratio H₂O/Me is from 2to
 3. 7. The process for the preparation and deposition of a vitreousfilm onto a substrate according to claim 1 where hydrolyzing of thealkoxide is accomplished in presence of an acid catalyst selected fromthe group consisting of mineral and organic acids with Ka of 0.1 to 3.8. The process for the preparation and deposition of a vitreous filmonto a substrate according to claim 7 where the reaction of hydrolyzingis in presence of an aqueous solution of HCl.
 9. The process for thepreparation and deposition of a vitreous film onto a substrate accordingto claim 7 where the reaction of hydrolyzing is in presence of aquantity of acid such that a molar ratio alkoxide/acid is from 1/0.001to 1/1.
 10. The process for the preparation and deposition of a vitreousfilm onto a substrate according to claim 9 where the molar ratio ofalkoxide to acid is preferably from 1/0.1 to 1/0.01.
 11. The process forthe preparation and deposition of a vitreous film onto a substrateaccording to claim 1 where the removal of the alcohol is made bysubjecting the sol to partial and controlled desolventization.
 12. Theprocess for the preparation and deposition of a vitreous film onto asubstrate according to claim 1 further comprising depositing onsubstrate by knife coating, dip coating or spin-coating.
 13. The processfor the preparation and deposition of a vitreous film onto a substrateaccording to claim 1 further comprising depositing on substrate byspin-coating.
 14. The process for the preparation and deposition of avitreous film onto a substrate according to claim 1 where final dryingis at a temperature of 20 to 500° C.
 15. A process for the preparationof a sol for the preparation of a vitreous film on a substratecomprising: preparing a solution in an aprotic solvent selected from thegroup consisting of acetone, tetrahydrofuran and dioxane of at least onemetal alkoxide corresponding to the formulaX_(m)—Me—(OR)_(n−m) where Me is a metal selected from the groupconsisting of Group 3°, 4° and 5° of the Periodic Table of Elements; nis the valence of Me; X is R₁ or OR₁, R₁ is the same as or differentfrom R, m is either zero or integer number equal to or lower than 3; Rand R₁ are hydrocarbon radicals with a number of carbon atoms equal toor lower than 12; adding a catalyst to the solution; hydrolyzing thesolution in the presence of the catalyst in a hydrolysis reaction byaddition of water to maintain a molar ratio of H₂O/Me between 0.5 and 5to obtain a clear hydrolysis reaction product; removing alcohol formedduring the hydrolysis reaction under reduced pressure and forming thesol in the aprotic solvent wherein an equal volume of the aproticsolvent replaces the alcohol removed.
 16. The process for thepreparation of a sol according to claim 15 wherein the alkoxide isselected from the group consisting of tetramethylorthosilicate,tetraethylorthosilicate, tetrapropylorthosilicate,tetrabutylorthosilicate, ethyltriethoxysilane, methyltrimethoxysilane,methyltriethoxysilane, and mixture thereof.
 17. The process for thepreparation of a sol according to claim 15 where the solution of thealkoxide or mixture of alkoxides in the aprotic solvent is from 30% to60% by weight.
 18. The process for the preparation of a sol according toclaim 15 wherein hydrolyzing of the alkoxide is accomplished by adding acontrolled quantity of water.
 19. The process for the preparation of asol according to claim 15 where the ratio H₂O/Me is from 1.5 to
 4. 20.The process for the preparation of a sol according to claim 19 where theratio H₂O/Me is from 2 to
 3. 21. The process for the preparation of asol according to claim 15 where hydrolyzing of the alkoxide isaccomplished in presence of an acid catalyst selected from the groupconsisting of mineral and organic acids with Ka of 0.1 to
 3. 22. Theprocess for the preparation of a sol according to claim 21 where thereaction of hydrolyzing is in the presence of an aqueous solution ofHCl.
 23. The process for the preparation of a sol according to claim 22where the reaction of hydrolyzing is in presence of a quantity of acidsuch that a molar ratio alkoxide/acid is from 1/0.001 to 1/1.
 24. Theprocess for the preparation of a sol according to claim 23 where themolar ratio of alkoxide to acid is preferably from 1/0.1 to 1/0.01. 25.The process for the preparation of a sol according to claim 15 where theremoval of the alcohol is made by subjecting the sol to partial andcontrolled desolventization.